Vehicles use a wide variety of transmissions for vehicle propulsion. Common types include manually shifted, multi-stage geared transmissions and automatic transmissions using internal power-operated (electrically or hydraulically) clutches, usually with a torque converter.
Another type of transmission which finds wide use in smaller vehicles, e.g., snowmobiles, all-terrain vehicles (ATVs) and small haulage and towing vehicles is an endless belt transmission, often referred to as a continuously variable transmission (CVT) since both output torque and speed vary substantially continuously, i.e., without gear shifts, over the entire speed range of the engine. A CVT is disclosed in U.S. Pat. No. 4,585,429 (Marier).
Since the operation of endless belt power transmission systems of the CVT type is by no means intuitive, it will be helpful to provide a detailed explanation of such systems before describing the invention even though such invention relates only to the system output pulley and the mechanism related thereto.
A CVT has two pulleys, namely, a flyweight pulley driven by the engine and an output pulley used to power an output shaft. An endless belt engages both pulleys.
A CVT transmission uses changing effective pulley diameters to change the torque available at the output shaft and the driven speed of such output shaft. Very high torque (at low rotational speed) is available as the vehicle moves from standstill while modest torque at high rotational speed is used for high speed propulsion.
Power transmission systems of this type automatically and continuously adjust torque and speed to provide a more-or-less constant horsepower drive system. (It will be recalled that horsepower is the product of torque, speed and some constant, the latter being a function of the units of measure of torque. The equation is HP=T.times.S.times.K.)
Considered in more detail, in a power transmission system of the CVT type, a flyweight pulley is directly coupled to and powered by the engine. The flyweight pulley has two disc-like pulley sheaves mounted in opposed relationship. Together, the sheaves form a pulley.
The flyweight pulley is constructed in such a way that the first sheave is axially fixed (as well as rotationally fixed) with respect to the engine drive shaft. The second sheave is axially movable with respect to the first. As the second sheave moves with respect to the first, the effective diameter of the flyweight pulley changes.
An endless belt engages the engine flyweight pulley and also engages an output pulley, the driven shaft of which powers the vehicle. The output pulley, of the type often referred to as a split pulley, also has an axially-fixed first sheave and a second sheave axially movable with respect to the first. In this type of transmission, horsepower is transmitted from the flyweight pulley to the output pulley because the sheaves apply pressure to the side edges of the belt. That is, the bottom surface of the belt does not contact the "root" of the pulley.
As engine speed increases somewhat above idle speed, the resulting modest centrifugal force overcomes spring force and the second sheave of the flyweight pulley moves toward the first sheave, thereby engaging the belt to move the vehicle from standstill to some low speed. At this low engine speed, the belt is engaged but, notably, the effective diameter of the flyweight pulley, which is then relatively small, does not change.
As engine speed is further increased, the resulting more aggressive centrifugal force overcomes a higher spring force in the output pulley and the second sheave moves further toward the first sheave, thereby increasing the effective diameter of the flyweight pulley. Since the belt is of fixed length, the effective diameter of the output pulley must necessarily decrease. This reduces the torque and increases the speed available at the output shaft driven by the output pulley.
From the foregoing, it is apparent that at vehicle startup, relatively high torque (at low speed) must transfer from the flyweight pulley through the belt to the output pulley and thence to the output drive shaft powering the snow-engaging belt (in a snowmobile) or the wheels of an ATV, to cite but two examples.
To transfer such torque, the movable sheave of the output pulley must be urged toward the fixed sheave to apply sufficient force to the edges of the belt to prevent belt slippage or at least prevent significant belt slippage. And, ideally, the amount of force applied to the belt by the output pulley (by movement of the movable sheave) should be generally proportional to the torque being transmitted. The objectives are to (a) provide enough pulley-engaging force to transmit torque, and (b) avoid using excessive force which can significantly reduce belt life and introduce unwanted inefficiency into the transmission system.
Some time ago, a mechanism known as a torque-responsive clutch was developed to meet this need in a CVT. Components of such a clutch include the sheaves of the output pulley and examples of such clutches are disclosed in U.S. Pat. Nos. 3,939,720 (Aaen et al.); 4,378,221 (Huff); 5,403,240 (Smith); 5,516,333 (Benson); 5,720,681 (Benson) and others.
A typical torque-responsive clutch has what is known as a helix, an example of which is shown in U.S. Pat. No. Des. 382,283 (Benson). Helix-contacting devices, plastic "buttons" or rollers, bear against the helix and urge the second sheave toward the belt. An example of a roller-type helix-contacting device is shown in U.S. Pat. No. Des. 367,023 (Benson).
Known torque-responsive clutches mount the helix-contacting devices in what might be termed an open operating environment as depicted in the Benson '023, '333 and '681 patents. Other embodiments mount such devices on the outside of a cylindrical can-like component sometimes referred to as a bearing carrier.
A disadvantage of such arrangements arises from the fact that the rotational speed of the torque-responsive clutch can be very high, i.e., on the order of several thousand revolutions per minute. Consequently, the centrifugal force is also high and breakage of the helix-contacting device can send shrapnel-like fragments flying away from the clutch and toward the vehicle operator or toward other operating components of the vehicle. Personal injury and/or mechanical damage may result.
Another disadvantage is that, since the helix-contacting device is on the outside of the bearing carrier, centrifugal force imposes tension on the device support pin. A crack or fracture could result.
Yet another disadvantage of helix-contacting devices of the non-rotating plastic button type is that the button riding on the helix surface creates significant friction and, therefore, inefficiency. And the friction is not constant. The static friction between the helix and the plastic button (that friction existing when there is no relative movement between the helix and the button) is higher than the dynamic friction, i.e., that friction existing when there is relative movement between the helix and the button. In an aggravated case, such difference in friction may be noticeable during operation of the vehicle.
A torque-responsive clutch which addresses disadvantages of known clutches would be an important advance in the art.